With the rapid development in semiconductor lighting industry, the electro-optical conversion efficiency of a LED is up to 70˜120 lm/W, which is far more than that of an Incandescent lamp and will exceed the efficiency of a Fluorescent lamp. Recently, LED has been widely used in various applications such as road lighting, landscape lighting, etc., and will be used in the area of residential lighting soon.
However, as a LED is typically operated only under DC power, the AC power from the main supply can be used to drive a LED only after the AC-DC conversion. For this reason, it is necessary to provide a power converting device for LEDs to be connected to the existing AC power supply network. Presently, each of the existing LED devices is supplemented with an individual power adapter connecting to an AC electric power socket. This way causes much higher costs due to the excessive use of multi-quantities of converting devices.
The LED lamp is a new kind of light source with advantages of energy saving, environment protecting, small size, multi-color and long life. But it is difficult to supply power to LEDs because of their sharp voltage-current characteristics. Unlike incandescent lamps, a LED lamp cannot be driven directly by unconverted AC power or unregulated DC power, since a small fluctuation in voltage would lead to a significant change in current and may even burn up the LED.
Moreover, there is a need to adjust LED luminosity in many applications. For example, dimming function is typically required in a LED reading lamp to provide different brightness and contrast. Two methods may be utilized to adjust the LED luminosity, that is, analog dimming method and PWM (Pulse Width Modulation) dimming method. The linear dimming may be easily implemented by analog method, but it has a disadvantage of color-shifting, thereby it is not suitable for the applications where fine and accurate dimming is required. Though the PWM dimming may solve this problem, it needs a special driving module.
Additionally, the existing cables for indoor distribution network typically carry voltages of 110V or 220V, according to the main power supply. Thus, they must be protected against electric shock for safe use. This results in a much higher cost comparing with a low-voltage DC distribution wires.